Method and system for transmitting a data frame of an electromagnetic telemetry signal to or from a downhole location

ABSTRACT

Methods, techniques, and systems for transmitting a data frame of an electromagnetic (EM) telemetry signal to or from a downhole location can be performed using multi-channel transmissions in which at least two different channels at two different frequencies are used to send a long header and the data frame. At least a portion of the long header is transmitted at a first frequency, and at least a portion of the data frame and any remaining portion of the long header not transmitted at the first frequency are transmitted at one or more additional frequencies different from each other and the first frequency. In one example, only two channels are used: one channel is used to transmit all of the long header, another channel is used to transmit all of the data frame, and either of the channels is used to transmit an identifier identifying the type of the data frame.

TECHNICAL FIELD

The present disclosure is directed at systems, methods, and techniquesfor transmitting a data frame of an electromagnetic telemetry signal toor from a downhole location.

BACKGROUND

The recovery of hydrocarbons from subterranean zones relies on theprocess of drilling wellbores. The process includes drilling equipmentsituated at surface, and a drill string extending from the surfaceequipment to a below-surface formation or subterranean zone of interest.The terminal end of the drill string includes a drill bit for drilling(or extending) the wellbore. The process also involves a drilling fluidsystem, which in most cases uses a drilling “mud” that is pumped throughthe inside of piping of the drill string to cool and lubricate the drillbit. The mud exits the drill string via the drill bit and returns tosurface carrying rock cuttings produced by the drilling operation. Themud also helps control bottom hole pressure and prevent hydrocarboninflux from the formation into the wellbore, which can potentially causea blow out at surface.

Directional drilling is the process of steering a well from vertical tointersect a target endpoint or follow a prescribed path. At the terminalend of the drill string is a bottom-hole-assembly (“BHA”) whichcomprises 1) the drill bit; 2) a steerable downhole mud motor of arotary steerable system; 3) sensors of survey equipment used inlogging-while-drilling (“LWD”) and/or measurement-while-drilling (“MWD”)to evaluate downhole conditions as drilling progresses; 4) means fortelemetering data to surface; and 5) other control processes such asstabilizers or heavy weight drill collars. The BHA is conveyed into thewellbore by a string of metallic tubulars (i.e. drill pipe). MWDequipment is used to provide downhole sensor and status information tosurface while drilling in a near real-time mode. This information isused by a rig crew to make decisions about controlling and steering thewell to optimize the drilling speed and trajectory based on numerousfactors, including lease boundaries, existing wells, formationproperties, and hydrocarbon size and location. The rig crew can makeintentional deviations from the planned wellbore path as necessary basedon the information gathered from the downhole sensors during thedrilling process. The ability to obtain real-time MWD data allows for arelatively more economical and more efficient drilling operation.

Some known MWD tools contain a sensor package to survey the wellbore andsend data back to surface using a telemetry method known aselectromagnetic (EM) telemetry. EM telemetry involves using an EMtelemetry tool to generate EM waves carrying encoded data and transmitthese waves from the wellbore through the surrounding formations, andusing surface receiving equipment to detect and decode the waves atsurface. The BHA metallic tubular is typically used as the dipoleantennae for the EM telemetry tool by dividing the drill string into twoconductive sections by an insulating joint or connector (“gap sub”)typically placed within the BHA, with the bottom portion of the BHA andthe drill pipe each forming an antennae for the dipole antennae. In EMtelemetry systems, a very low frequency alternating current is drivenacross the gap sub. The sub is electrically isolated (“nonconductive”)at the insulating joint, effectively creating an insulating break(“gap”) between the portion of the drill string below the gap and theportion above the gap, which extends all the way up to the surface. Thelower part below the gap typically is set as a ground but the polarityof the members can be switched. The low frequency AC voltage andmagnetic reception is controlled in a timed/coded sequence to energizethe earth and create a measurable voltage differential between thesurface ground and the top of the drill string. The EM signal whichoriginated across the gap is detected at surface and measured as adifference in the electric potential from the drill rig to varioussurface grounding rods located about the drill site.

Typically, a sinusoidal waveform is used as a carrier signal for thetelemetry data. The MWD tool comprises a downhole modulator which canuse one of a number of encoding or modulation schemes to encode thetelemetry data onto a carrier waveform. The three key parameters of aperiodic waveform are its amplitude (“volume”), its phase (“timing”) andits frequency (“pitch”). Any of these properties can be modified inaccordance with a low frequency signal to obtain a modulated signal.Frequency-shift keying (“FSK”) is a frequency modulation scheme in whichdigital information is transmitted through discrete frequency changes ofa carrier wave. The simplest FSK is binary FSK (“BFSK”). BFSK uses apair of discrete frequencies to transmit binary information. Amplitudeshift keying (“ASK”) conveys data by changing the amplitude of thecarrier wave; phase-shift keying (“PSK”) conveys data by changing, ormodulating, the phase of a reference signal (the carrier wave). It isknown to combine different modulation techniques to encode telemetrydata.

The choice of modulation scheme uses a finite number of distinct signalsto represent digital data. PSK uses a finite number of phases, whereineach phase is assigned a unique pattern of binary digits, or “symbols”,and wherein the symbols together form a defined symbol set. Usually,each phase encodes an equal number of bits. Each pattern of bits formsthe symbol that is represented by the particular phase. A surfacedemodulator contains the same symbol set used by the downhole modulator,and determines the phase of the received signal and maps it back to thesymbol it represents, thus recovering the original data.

SUMMARY

According to a first aspect, there is provided a method for transmittinga data frame of an electromagnetic (EM) telemetry signal to or from adownhole location. The method comprises transmitting at least a portionof a long header at a first frequency, wherein the EM telemetry signalcomprises the long header and the data frame; and transmitting at leasta portion of the data frame and any remaining portion of the long headernot transmitted at the first frequency at one or more additionalfrequencies different from each other and from the first frequency,wherein the first frequency and the one or more additional frequenciesare sufficiently separated to permit demodulation of the portion of thelong header transmitted at the first frequency and the portion of thedata frame and any remaining portion of the long header transmitted atthe one or more additional frequencies.

All of the long header may be transmitted at the first frequency and allof the data frame may be transmitted at a second frequency.

In one aspect,

${{f^{\prime}\left( {1 + \frac{1}{c^{\prime}}} \right)} \leq {f\left( {1 - \frac{1}{c}} \right)}},$wherein f is one of the first and second frequencies, f′ is the other ofthe first and second frequencies, f>f′, c is cycles per bit transmittedat f, and c′ is cycles per bit transmitted at f′. Additionally oralternatively, c may equal c′.

f′ may be the first frequency and f may be the second frequency.

The method may further comprise transmitting an identifier (ID), whichvaries with the content of the data frame, at the first or secondfrequencies.

All of the ID and of the data frame may be transmitted at the secondfrequency.

The long header may be transmitted prior to the ID.

At least a portion of the long header may be transmitted simultaneouslywith a previous data frame.

All or less than all of the previous data frame may be transmitted priorto transmission of the ID.

The previous data frame and the long header may complete transmittingsimultaneously.

At least a portion of the long header may be transmitted simultaneouslywith the ID.

The long header may complete transmitting during transmission of thedata frame.

All of the long header and the ID may be transmitted at the firstfrequency and all of the data frame may be transmitted at the secondfrequency.

All or less than all of a previous data frame may be transmitted priorto transmission of the data frame.

The previous data frame and the ID may complete transmittingsimultaneously.

The frame may be transmitted to the downhole location, and the methodmay further comprise pausing between transmitting the ID and the dataframe.

The frame may be transmitted from the downhole location and sent tosurface.

The data frame may comprise data obtained during a survey andtransmission of the long header may commence during an idle periodimmediately prior to acquiring the survey.

According to another aspect, there is provided a method for transmittinga data frame of an electromagnetic (EM) telemetry signal from a downholelocation to surface. The method comprises: during an idle periodfollowing cessation of pumping drilling fluid, transmitting at least aportion of a long header, wherein the EM telemetry signal comprises thelong header and the data frame; acquiring a survey during a surveyacquisition period that immediately follows the idle period, wherein thedata frame comprises results of the survey; encoding results of thesurvey as the data frame; and transmitting the data frame and anyportion of the long header not transmitted during the idle period.

The method may further comprise transmitting an identifier (ID), whichvaries with the content of the data frame.

All of the long header may be transmitted during the idle period.

All of the ID may be transmitted during the idle period.

According to another aspect, there is provided a non-transitory computerreadable medium having encoded thereon statements and instructions tocause a processor to perform any of the aspects of the methods describedabove.

According to another aspect, there is provided a downhole telemetry toolfor transmitting a frame of an electromagnetic (EM) telemetry signalfrom a downhole location. The tool comprises a gap sub assembly; an EMcarrier frequency signal generator for generating an EM carrier waveacross an electrically isolated gap of the gap sub assembly; and anelectronics subassembly communicative with the signal generator andcomprising a downhole processor and a memory containing an encoderprogram code executable by the downhole processor to perform any of theaspects of the methods described above.

This summary does not necessarily describe the entire scope of allaspects. Other aspects, features and advantages will be apparent tothose of ordinary skill in the art upon review of the followingdescription of specific embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings, which illustrate one or more exemplaryembodiments:

FIG. 1 is schematic side view of a multi-channel EM telemetry system inoperation, according to one embodiment.

FIG. 2 is a schematic side view of components of a downhole EM telemetrytool of the EM telemetry system according to one embodiment.

FIG. 3 is a schematic block diagram of components of an electronicssubassembly of the downhole EM telemetry tool.

FIG. 4 is a schematic block diagram of the components of the downhole EMtelemetry tool.

FIG. 5 is a schematic block diagram of surface components of themulti-channel EM telemetry system according to one embodiment.

FIG. 6 is a schematic block diagram of surface components of themulti-channel EM telemetry system according to another embodiment.

FIG. 7 is a flow chart of steps performed by the surface components ofthe multi-channel EM telemetry system to receive and decode themulti-channel EM telemetry signal transmitted by the downhole EMtelemetry tool.

FIG. 8 is a graph of a first downhole waveform of a first telemetrysignal.

FIG. 9 is a graph of a second downhole waveform of a second telemetrysignal.

FIG. 10 is a graph of a multi-channel EM telemetry signal that is thecombination of the first and second downhole waveforms, as transmittedby the downhole EM telemetry tool.

FIG. 11 is a graph of the multi-channel EM telemetry signal as receivedby the surface components.

FIG. 12 is a graph of first telemetry signal separated from the EMtelemetry signal by the surface components.

FIG. 13 is a graph of second telemetry signal separated from the EMtelemetry signal by the surface components.

FIG. 14 is a graph of a downhole reference frequency sweep waveformhaving a continuously increasing frequency according to one embodiment.

FIG. 15 is a graph of the downhole reference frequency sweep waveform ofFIG. 14 as received by a surface receiver.

FIG. 16 is a graph plotting the amplitude against frequency range of thereceived downhole frequency sweep waveform of FIG. 15.

FIG. 17 is a graph of a downhole reference frequency sweep waveformhaving discrete frequency steps according to another embodiment.

FIG. 18 is a graph of the downhole reference frequency sweep waveform ofFIG. 17 as received by a surface receiver.

FIG. 19 is a graph plotting the amplitude against frequency range of thereceived downhole frequency sweep waveform of FIG. 18.

FIG. 20 is a block diagram of an EM signal transmitter of the EMtelemetry system, according to one embodiment.

FIG. 21(a) depicts an exemplary waveform of a header portion of a frame.

FIGS. 21(b) and (c) depict exemplary bitstreams transmitted seriallyalong a single channel.

FIGS. 22(a)-(d) depict four bitstreams transmitted using multiplechannels, according to four exemplary embodiments.

FIG. 23(a) depicts mud flow vs. time during a drilling operation, whileFIGS. 23(b) and 23(c) depict exemplary bitstreams transmitted during thedrilling operation.

FIGS. 24(a) and (b) and FIGS. 25(a)-(c) depict various methods fortransmitting a frame to or from a downhole location, according toadditional exemplary embodiments.

FIGS. 26(a)-(f) depict exemplary waveforms generated in accordance withthe embodiment of FIG. 25(a).

DETAILED DESCRIPTION

Directional terms such as “top,” “bottom,” “upwards,” “downwards,”“vertically,” and “laterally” are used in the following description forthe purpose of providing relative reference only, and are not intendedto suggest any limitations on how any article is to be positioned duringuse, or to be mounted in an assembly or relative to an environment.

An EM signal generated by a downhole EM signal transmitter needs to havea sufficient strength that the signal is still detectable at surface bya surface EM signal receiver despite considerable attenuation of thetransmitted signal as the signal travels long distances through theearth's formations.

The embodiments described herein generally relate to a multi-channel EMtelemetry system which comprise a downhole EM telemetry tool thatcombines multiple EM waveforms each representing a separate channel oftelemetry data into a combined waveform and transmits this combinedwaveform to surface in a single EM transmission, and which alsocomprises a surface receiver that receives the combined waveform anddecodes the waveform into the separate channels of telemetry data. It isexpected that a high overall efficiency of data transmission can beachieved by sending multiple channels of telemetry data in a single EMtransmission comprising the combined waveform.

Referring to FIG. 1, there is shown a schematic representation of an EMtelemetry system in which various embodiments can be employed. Downholedrilling equipment including a derrick 1 with a rig floor 2 and drawworks 3 facilitates rotation of the drill pipe 6 into the ground 5. Thedrill pipe 6 is enclosed in casing 8 which is fixed in position bycasing cement 9. Bore drilling fluid 10 is pumped down drill pipe 6 andthrough an electrically isolating gap sub assembly 12 to drill bit 7.Annular drilling fluid 11 is then pumped back to the surface and passesthrough a blow out preventer (“BOP”) 4 positioned above the groundsurface. The gap sub assembly 12 may be positioned at the top of theBHA, with the BHA and the drill pipe 6 each forming antennae for thedipole antennae. The gap sub assembly 12 is electrically isolated(“nonconductive”) at its center joint effectively creating an insulatingbreak, known as a gap, between the bottom of the drill string with theBHA and the larger top portion of the drill string that includes therest of the drill pipe 6 up to the surface. A very low frequencyalternating electrical current 14 is generated by an EM signaltransmitter 13 and driven across the gap sub assembly 12 to generatediscrete pulses which carry encoded telemetry data (“EM telemetrytransmission”). The low frequency AC voltage and magnetic reception iscontrolled in a timed/coded sequence to energize the earth and create anelectrical field 15. Communication cables 17 transmit the measurablevoltage differential from the top of the drill string and varioussurface grounding rods 16 located about the drill site to surfacereceiving and processing equipment 18 which receives and processes theEM telemetry transmission. The grounding rods 16 are generally randomlylocated on site with some attention to site operations and safety. TheEM telemetry transmission is decoded and a receiver box communicationcable 19 transmits the data received to a computer display 20 afterdecoding, thereby providing measurement while drilling information tothe rig operator.

Referring now to FIG. 2, an EM telemetry system 30 according to oneembodiment comprises a multi-channel downhole EM telemetry tool 32 andthe surface receiving and processing equipment 18 (not shown in FIG. 2).The EM telemetry tool 32 generally comprises the gap sub assembly 12,the EM signal transmitter 13, and an electronics subassembly 36. Theelectronics subassembly 36 houses sensors for taking downholemeasurements as well as a processor and memory which contains programcode executable by the processor to encode the sensor measurements intomultiple channels of telemetry data each with a separate EM waveform,combine the separate EM waveforms into a combined waveform, and sendcontrol signals to the EM signal transmitter 13 to transmit the combinedwaveform to surface. The surface receiving and processing equipment 18comprise equipment to receive the combined waveform, filter and processthe waveform, and decode the waveform into the telemetry data.

The gap sub assembly 12 comprises an electrically conductive femalemember 37 comprising a female mating section and an electricallyconductive male member 40 comprising a male mating section. The malemating section is matingly received within the female mating section andelectrically isolated therefrom by an electrical isolator 46. Theelectrical isolator 46 comprises electrical insulating material that ispositioned in between the male and female mating sections. Theelectrical isolator 46 thereby electrically isolates the male member 40from the female member 37, and the male member 40, female member 37 andelectrical isolator 46 together function as the gap sub assembly 12 forEM telemetry.

Referring to FIG. 3, the electronics subassembly 36 includes sensors fortaking various downhole measurements, and encoding and processingequipment for encoding the measurements and other information(collectively “telemetry data”) into a telemetry signal for transmissionby the EM signal transmitter 13. More particularly, the electronicssubassembly 36 comprises a tubular housing (not shown) and the followingcomponents housed inside the housing: a directional and inclination(D&I) sensor module 50; drilling conditions sensor module 52; a maincircuit board 54 containing a master processing unit (MPU or otherwisereferred to as the “downhole processor”) 56 having an integrated (shown)or separate (not shown) data encoder, a memory 58 having stored thereonprogram code executable by the downhole processor 56, one or more poweramplifiers (not shown); and a battery stack 60. The downhole processor56 can be any suitable processor known in the art for EM tools, and canbe for example, a dsPIC33 series MPU. The power amplifiers can be, forexample, a power MOSFET H-bridge design configured to transmit data.

The D&I sensor module 50 comprises three axis accelerometers, three axismagnetometers and associated data acquisition and processing circuitry.Such D&I sensor modules are well known in the art and thus are notdescribed in detail here.

The electronics subassembly 36 includes sensors and circuitry for takingvarious measurements of borehole parameters and conditions includinggamma, temperature, pressure, shock, vibration, RPM, and directionalparameters. Such sensors and circuitry are also well known in the artand thus are not described in detail here.

The main circuit board 54 can be a printed circuit board with electroniccomponents soldered on the surface of the board 54. The main circuitboard 54 and the sensor modules 50, 52 are secured on a carrier device(not shown) which is fixed inside the electronics subassembly housing byend cap structures (not shown). The sensor modules 50, 52 are eachelectrically communicative with the main circuit board 54 and sendmeasurement data to the downhole processor 56.

As will be described below, the memory 58 contains encoder program codethat can be executed by the downhole processor 56 to perform a method ofencoding and transmitting a multi-channel EM telemetry signal using acombined waveform that is a combination of two or more separatewaveforms each representing a separate telemetry channel. Referring toFIG. 4, the downhole processor 56 reads raw measurement data from thesensor modules 50, 52 and encodes this raw data into an encoded digitalbitstream (step 70). Then the downhole processor 56 separates theencoded digital bitstream into two or more separate digital bitstreams,which in the embodiment shown in FIG. 4 comprises a first bitstream anda second bitstream (step 72). Then, the downhole processor 56 convertseach digital bitstream into a separate waveform, namely a first waveformand a second waveform (otherwise referred to as “first telemetrychannel” and “second telemetry channel”); this conversion involves usinga selected digital modulation technique to modulate the waveforms,wherein the frequency of each waveform being modulated is assigned aunique value or a unique, non-overlapping range of values, i.e. a valueor range of values that is different than the value or range of valuesof the corresponding parameter(s) of the other waveforms (step 74). Thenthe downhole processor 56 combines each separate waveform into acombined waveform (step 76), and then sends a control signal to thesignal transmitter 13 to transmit an EM telemetry signal comprising thecombined waveform (otherwise referred to as “carrier wave”) across thegap sub 12 (step 78).

Various digital modulation techniques known in the art can be used toencode each separate waveform, such as ASK, PSK, FSK, binary phase-shiftkeying (BPSK), quadrature phase shift keying (QPSK) or any combinationof these or other individual modulation techniques as is known in theart. As one (or more) of the amplitude, frequency and phase of theseparate waveforms is set at a unique value, the plurality of separatewaveforms encoded by one or more of these techniques can be superimposedto form one combined waveform (carrier wave) for transmission tosurface. As a result, one EM signal comprising the carrier wave composedof two or more telemetry channels can be transmitted to surface.

In one example and referring to FIGS. 8 to 10, each separate waveformcan be modulated using BPSK with different frequencies then combinedinto the combined waveform. FIG. 8 shows a first downhole waveform forthe first channel, having a maximum amplitude of 1.0, a frequency of 6Hz, and a phase shift of 180 degrees at time 0.5. FIG. 9 shows a seconddownhole waveform for the second channel, having a maximum amplitude of1.0, a frequency of 12 Hz, and a phase shift of 180 degrees at time 0.5.FIG. 10 shows a combined downhole waveform representing the combinationof the first and second downhole waveforms.

In another example (not shown), each separate waveform can be modulatedusing ASK with different frequencies. In this modulation technique, thepower amplifiers can be used to modulate the amplitudes of each waveforminstead of or in addition to the processor performing a digital ASKmodulation.

Referring now to FIG. 20, there is shown a block diagram of the EMsignal transmitter 13 communicatively coupled to the gap sub 12. The EMsignal transmitter 13 is configured to generate bipolar EM pulses tocarry the telemetry signal encoded by the modulation techniquesdiscussed above. Bipolar EM pulses mean pulses that have either apositive or negative amplitude. The EM signal transmitter 13 comprisesan H-bridge circuit 106 which is used to switch the polarity of the EMpulse, a step-up/step-down voltage regulator 108, a battery 110, and asignal generator 112. The voltage regulator 108 comprises separatestep-up and step-down voltage regulators 114, 116, a pulse widthmodulation driver (“PWM driver”) 118, and diodes 120 that electricallyconnect the outputs of these voltage regulators 114, 116 to the input ofthe PWM driver 118. The output of the PWM driver 118 is used as theoutput of the voltage regulator 108. The H-bridge circuit 106 enables avoltage to be applied across a load in either direction, and comprisesfour switches of which one pair of switches can be closed to allow avoltage to applied in one direction (“positive pathway”), and anotherpair of switches can be closed to allow a voltage to be applied in areverse direction (“negative pathway”). In the H-bridge circuit 106 ofthe EM signal transmitter 13, switches S1, S2, S3, S4 are arranged sothat the part of the circuit with switches S1 and S4 is electricallycoupled to one side of the gap sub 12 (“positive side”), and the part ofthe circuit with switches S2 and S3 are electrically coupled to theother side of the gap sub 12 (“negative side”). Switches S1 and S3 canbe closed to allow a voltage to be applied across the positive pathwayof the gap sub 12 to generate a positive polarity pulse, and switches S2and S4 can be closed to allow a voltage to be applied across thenegative pathway of the gap sub 12 to generate a negative polaritypulse.

One end of the H-bridge circuit 106 is grounded and the other end iselectrically coupled to the output of the voltage regulator 108. Thevoltage regulator 108 in turn is electrically coupled to the battery 110as well as to the signal generator 112 and the processor 56. Morespecifically, the output of the signal generator 112 is electricallycoupled to the input of the PWM driver 118, and the processor 56 outputscontrol signals to the step-up regulator 114 and the step-down regulator116 to control which one is active at any given time. As described infurther detail below, by using the step-up and step-down voltageregulators 114,116, the voltage regulator 108 is able to adjust thestrength of the EM transmission to surface.

The signal generator 112 is communicative with the downhole processor56, and serves to receive the encoded telemetry signal from the downholeprocessor 56, and determine the appropriate control signals to send tothe voltage regulator 108. In particular, the signal generator 112 willsend voltage output and timing control signals to the voltage regulator108 to output a voltage at a certain voltage level and timing (or at thefixed voltage level if there is no ASK). The processor 56 sends polaritycontrol signals to the H-bridge circuit 106 to close the appropriateswitches to create a positive polarity EM pulse or a negative polarityEM pulse in conjunction with the output of the signal generator 112.When the modulation technique involves timing shift keying, the signalgenerator 112 and the processor 56 are configured to send the controlsignals at a specific timing interval pattern in accordance with thetiming shift keying modulation scheme.

The signal generator 112 is capable of generating a pulse of differentwidths (frequencies). For example, the signal generator can generate awide pulse using a very low frequency, e.g. below 2 Hz, and inparticular, between 0.5 and 0.25 Hz. Transmitting at such lowfrequencies can be useful to take advantage of a natural bias towardslower frequencies in the Earth.

Referring now to FIG. 5, the surface receiving and processing equipment18 receives the carrier wave and decodes the combined waveform torecover each separate telemetry channel; the carrier wave sent by the EMtelemetry tool 32 shown in FIG. 10 will have attenuated as the carrierwave travels through the Earth, and FIG. 11 shows the carrier wave asreceived by surface receiving and processing equipment 18. The telemetrychannels can then be converted back into the measurement data for use bythe operator. As will be described in detail below, the surfacereceiving and processing equipment 18 will have stored thereon ademodulation technique corresponding to the selected modulationtechnique used by the EM telemetry tool 32 and the unique phase orfrequency value of each separate waveform of the carrier wave used bythe downhole EM telemetry tool 32 to encode the separate waveforms, sothat the carrier wave can be decoded to obtain the telemetry data.

The surface receiving and processing equipment 18 comprises a surfacereceiver 80 and a decoder 82. The surface receiver 80 comprises apreamplifier 84 electrically coupled to the communication cables 17 toreceive and amplify the EM telemetry transmission comprising the carrierwave, a band pass filter 86 communicative with the preamplifier 84configured to filter out unwanted noise in the transmission, and ananalog to digital converter (ADC) 88 communicative with the band passfilter 86 to convert the analog carrier wave into a digital signal. Suchpreamplifiers, band pass filters, and A/D converters are well known inthe art and thus are not described in detail here. For example, thepreamplifier can be an INA118 model from Texas Instruments, the ADC canbe an ADS1282 model from Texas Instruments, and the band pass filter canbe an optical band pass filter or an RLC circuit configured to passfrequencies between 0.1 Hz to 20 Hz.

The decoder 82 in one embodiment is a general purpose computercomprising a central processing unit (CPU and herein referred to as“surface processor”) and a memory having decoder program code executableby the surface processor to perform various decoding functions,including digital signal filtering and separation, digital signalprocessing, digital signal recombination, and digitalsignal-to-telemetry data decoding. Instead of using the surfaceprocessor to perform all of the decoding functions, separate hardwarecomponents can be used to perform one or more of the decoding functions;for example, an application-specific integrated circuit (ASIC) orfield-programmable gate arrays (FPGA) can be used to perform the digitalsignal processing in a manner as is known in the art (FIG. 5 showsdecoding steps performed by the surface processor as blocks within thedecoder 82, although these blocks can alternatively represent separatehardware components that perform the same functions).

Referring to FIG. 7, the decoder 82 receives the carrier wave that hasbeen digitized, filtered and amplified by the surface receiver 80 (step89), and performs each of the following decoding functions in thefollowing sequence (these decoding functions are stored as program codeon the memory of the computer and are executable by the surfaceprocessor):

Digital Filtering (step 90): When each of the different waveforms isencoded by a modulation technique at a unique frequency, the programcode comprises a series of band pass filters that are used to separatedifferent bands (frequency signals) from the carrier wave. Moreparticularly, each band pass filter is configured to pass one of theunique frequency bands corresponding to one of the separate waveformsand severely attenuate all other frequencies, such that the waveformcorresponding to this unique frequency band can be separated from thecarrier wave.

Digital Signal Processing (Step 92): Each separated waveform is in abitstream form and is then subjected to a series of digital processingtreatments known in the art, such as automatic gain control (AGC) tonormalize the signal amplitude, synchronization to find the phase andtiming differences between incoming signals and local oscillationsignals, and demodulation and decoding to recover binary bits. Suchdigital processing treatments are known in the art of digital signalprocessing and thus are not described in detail here. Then, eachseparate waveform is demodulated back into the corresponding measurementdata bitstream using a demodulation technique that is configured tocorrespond specifically to the modulation technique used to encode theseparate measurement data bitstreams into the separate waveforms. Usingthe example shown in FIGS. 8 to 10 but now referring to FIGS. 12 to 13,the demodulation technique will determine the phase of each separatewaveform, and map the waveform back to the symbol it represents, thusrecovering the original data bitstream (FIG. 12 shows the first waveformand FIG. 13 shows the second waveform). Such demodulation techniques arewell known in the art and thus are not described in detail here.

Digital Signal Recombination (Step 94): After demodulation, the separateraw measurement data streams are recombined back into the single rawmeasurement data stream that existed at step 89. Once the single rawmeasurement data bitstream has been recombined, the data can be decodedand viewed on the computer display 20 or manipulated by the operatorinto a useful form for display (step 96).

Instead of recombining all of the separate measurement data streams intothe single measurement data stream, a subset of separate data stream canbe combined for display to the operator, or each individual data streamcan be processed and displayed for the operator.

According to an alternative embodiment and referring to FIG. 6, thesurface receiver 80 and decoder 82 have been modified such that allsignal filtering is performed by the surface receiver 80 on the analogwaveforms, prior to digitization. More particularly, the surfacereceiver 80 employs narrow band hardware filters 98 of differentfrequency ranges to separate out each separate analog waveform from theanalog carrier wave. Multiple ADCs are then used to convert the multipleanalog waveforms into digital signals. In contrast, the surface receiver80 and decoder 82 according to FIG. 5 perform analog signal filtering ofthe analog carrier wave, as well as digital signal filtering of thedigitized carrier wave. Here, only one ADC is used to convert thecarrier wave into digital form, and digital bandpass filtering isperformed by software rather than by hardware.

Optionally, the decoder 82 can also execute an algorithm whichcompensates for attenuation caused by the low pass filtercharacteristics of the Earth and other conditions of the drill site.This attenuation compensation algorithm can be constructed in accordancewith one of the methods illustrated in FIGS. 14 to 19. In a first methodshown in FIGS. 14 to 16, a frequency sweep waveform 99 having aconsistent reference amplitude and a continuously increasing frequencyover a determined time period can be transmitted by the EM telemetrytool 32 during an idle time in the drilling so that the drilling processis not interrupted. As can be seen in FIG. 14, the reference amplitudeis set to match the amplitude of each separate waveform used by the EMtelemetry tool 32 to produce the combined waveform, and the frequencycan be set to increase at a selected rate. As can be seen in FIG. 15,the frequency sweep waveform 100 received at surface will attenuateafter having travelled through the Earth to the surface receiver 80. Theattenuation rate will tend to increase with increasing frequency,wherein the value of the attenuation rate will vary with the physicalproperties and operating conditions of the drill site. As can be seen inFIG. 16, the amplitude of the received frequency sweep waveform can beplotted over the frequency range to produce an attenuation curve 101.This attenuation curve 101 is stored on the DSP 92 and is applied to thereceived waveforms to compensate for the attenuation caused by theEarth.

According to another embodiment, other frequency sweep waveforms 102 and103 and an attenuation curve 104 produced from this frequency sweepwaveform are shown in FIGS. 17 to 19, respectively. In this frequencysweep waveform 102, the frequency is held constant for a short period oftime, then changed in discrete steps. FIG. 17 shows this frequency sweepwaveform as transmitted by the EM telemetry tool 32, and FIG. 18 showsan attenuated frequency sweep waveform 103 as received by the surfacereceiver which has been attenuated travelling through the Earth. FIG. 19shows a stepped attenuation curve over frequency. For the frequencies ofthe first and second downhole waveforms shown in FIGS. 8, 9 (6 and 12Hz), the expected surface amplitude levels would approximately be 0.1and 0.03 based on downhole amplitudes of 1.0.

Reducing Header Delays

Measurement data bitstreams are typically encoded into frames (“dataframes”) for transmission. Periodically transmitted with the data framesin the bitstreams are header portions, comprising one or both of longheaders and identifiers (IDs). The long headers are fixed andaccordingly are independent of the content of the data frames; they areused to synchronize communications between the EM signal transmitter 13and receiver 80. The IDs indicate the type of data comprising the dataframe, and accordingly vary with the type of data frame sent. Forexample, the data frame may comprise data that

-   -   (a) is obtained during a survey (this type of data frame is a        “survey frame”);    -   (b) is obtained during sliding drilling, which is powered using        a mud motor and without rotating the drill string at surface        (this type of data frame is a “sliding frame”);    -   (c) is obtained during drilling powered using a rotary table or        top drive (this type of data frame is a “rotating frame”); or    -   (d) indicates a change in telemetry type, speed, amplitude,        configuration, or of some other characteristic potentially        useful to the receiver 80 (this type of data frame is a “status        frame”). Unlike the survey, sliding, and rotating frames, the        status frame is triggered by particular events that occur during        drilling rather than by the type of drilling being performed or        the purpose of the measurements being made.

The data in the data frame comprises a series of data packets. The dataframe also includes error detection and correction bits, such as CRCbits.

In an embodiment in which the transmitter 13 is able to send survey,sliding, rotating, and status frames, the ID may, for example, be threebits long, and be “000” when a survey frame is sent, “010” when asliding frame is sent, “100” when a rotating frame is sent, and “101”when a status frame is sent. Other possible values for the ID can bereserved for other types of frames.

As mentioned above, the long headers are fixed and are used tosynchronize communications between the EM signal transmitter 13 andreceiver 80. Synchronization is useful, for example, when determiningthe phase of a carrier wave or pulse timing during demodulation. Thelong header is transmitted once every several frames to permit periodicsynchronization. Conventional long headers are relatively large atapproximately 15 to 20 bits so that the receiver 80 is able todistinguish them from the data frames and to reduce the likelihood thatone of the data frames and one of the long headers are inadvertentlyidentical. FIG. 21(a) depicts an exemplary one of the long headers andIDs. The EM signal transmitter 13 and the surface receiver 80 both haveaccess to a configuration file, which details the different types ofdata frames that can be sent and the different long headers and IDs thatmay be used; for example, the configuration file may contain informationdescribing the timing, frequency, signal processing method, and datastructure of the different possible data frames.

FIG. 21(b) depicts an exemplary measurement data bitstream comprising aplurality of data frames transmitted serially in one channel along asingle carrier wave. Periodically, the header portion comprising thelong header and ID is re-transmitted; when sent, it is transmittedbetween the end of one data frame and the beginning of another. In FIG.21(b), the period during which the long header and ID are transmitted isa period of delay during which no measurement data can be transmitted(“header delay”). The header delay reduces the effective data transferrate between the transmitter 13 and receiver 80. In the bitstream ofFIG. 21(b), the long header is transmitted after transmission of anentire one of the data frames.

FIG. 21(c) shows another exemplary measurement data bitstream in whichless than all of a data frame is transmitted before the long header istransmitted; that is, transmitting the long header interruptstransmitting a data frame. Notwithstanding this difference with thebitstream of FIG. 21(b), the period during which the long header and IDare transmitted in the bitstream of FIG. 21(c) also reduces theeffective data transfer rate between the transmitter 13 and the receiver80.

To increase the effective data transfer rate between the transmitter 13and the receiver 80 relative to the bitstreams of FIGS. 21(b) and (c),the EM telemetry system 30 can be used to transmit the long header anddata frame using at least two different channels at two differentfrequencies. On at least one of the channels the transmitter 13transmits the data frame, while on at least another of the channels thetransmitter 13 transmits at least part of the long header. Theembodiments depicted in FIGS. 22 through 26 transmit all of the dataframe on one channel, which is hereinafter referred to as the “datachannel”; they transmit all of the long header on another channel, whichis hereinafter referred to as the “header channel”. The ID istransmitted on either of the channels. Transmitting the long header andthe data frame on separate channels allows the length of the long headerto be decreased compared to when the long header and data frame aretransmitted on the same channel, since the receiver 80 is able to assumethat any transmission received on the header channel is the long headerinstead of having to determine from the content of a bitstream whetherthat bitstream represents the data frame or long header. For example, inthe depicted embodiments the length of the long header can be reducedfrom approximately 15 to 20 bits to approximately 7 bits. Additionally,in embodiments in which the data channel and the header channel are usedto simultaneously transmit a data frame and a long header, respectively,the effective data transfer rate is increased relative to when only asingle channel is used to transmit both the data frame and long header.

While the EM signal transmitter 13 used in the examples discussed belowis downhole while the receiver 80 is at surface, in alternativeembodiments the transmitter 13 and receiver may be downhole, or thetransmitter 13 may be at surface and the receiver may be downhole.

Referring now to FIG. 24(a), there is shown a method 2400 fortransmitting the data frame of the EM telemetry signal according to oneembodiment. The downhole processor 56 begins performing the method 2400at step 2402 and then proceeds to step 2404 where it transmits at leastpart of the long header, at a first frequency, which is the frequency ofthe carrier wave used for the header channel. The downhole processor 56then proceeds to step 2406 where it transmits at least a portion of thedata frame and any remaining portion of the long header not transmittedat the first frequency at one or more additional frequencies differentfrom each other and from the first frequency. The first frequency andthese one or more additional frequencies are sufficiently separated topermit demodulation of the portion of the long header transmitted at thefirst frequency and the portion of the data frame and any remainingportion of the long header transmitted at the one or more additionalfrequencies. In the embodiments discussed below, the remaining portionof the EM telemetry signal comprises all of the data frame and istransmitted at a second frequency that is the frequency of the carrierwave used for the data channel. Following transmission at step 2406, theprocessor 56 proceeds to step 2408 and the method 2400 ends.

In one embodiment, the frequencies of the carrier waves used to transmitthe measurement data bitstreams on the data and header channels areseparated by a minimum number of Hz to facilitate relatively error freedemodulation of the bitstreams. The following analysis aids indetermining proper levels of frequency separation between the data andheader channels. This analysis also applies generally to any two-channeltransmission performed by the system 30.

Suppose that f is the carrier frequency of the data channel, f′ is thecarrier frequency of the header channel, c is the chosen cycles per bitfor the data channel, and c′ is the chosen cycles per bit for the headerchannel; without loss of generality, f>f′. Additionally, c and c′>1.

The bandwidth occupied by the data channel is given by

$\begin{matrix}{B_{1} \in {\left( {{f\left( {1 - \frac{1}{c}} \right)},{f\left( {1 + \frac{1}{c}} \right)}} \right).}} & (1)\end{matrix}$

The bandwidth for the header channel is

$\begin{matrix}{B^{\prime} \in {\left( {{f^{\prime}\left( {1 - \frac{1}{c^{\prime}}} \right)},{f^{\prime}\left( {1 + \frac{1}{c^{\prime}}} \right)}} \right).}} & (2)\end{matrix}$

Therefore, in order not to incur bandwidth conflict between the twochannels:

$\begin{matrix}{{f^{\prime}\left( {1 + \frac{1}{c^{\prime}}} \right)} \leq {{f\left( {1 - \frac{1}{c}} \right)}.}} & (3)\end{matrix}$

From Equation (3), it can be seen that c≠1.

If the cycles per bit chosen for the two channels are identical, whichis typical in EM telemetry, Equation (3) can be simplified as

$\begin{matrix}{f^{\prime} \leq {{f\left( \frac{c - 1}{c + 1} \right)}.}} & (4)\end{matrix}$

In one embodiment, if cycles per bit for both channels c is three, then

$f^{\prime} \leq {\frac{f}{2}.}$Consequently, in this embodiment the maximum frequency for the headerchannel is

$\frac{f}{2}.$

In another embodiment, if cycles per bit for both channels c is two,then

$f^{\prime} \leq {\frac{f}{3}.}$Consequently, in this embodiment the maximum frequency for the headerchannel is

$\frac{f}{3}.$

While the frequency used for the header channel is less than that usedfor the data channel in the examples above, in alternative embodimentsthe frequency used for the header channel may be higher than that usedfor the data channel.

Referring now to FIG. 25(a), there is shown one embodiment of the method2400 for transmitting a data frame of the EM telemetry signal. Theembodiment shown in FIG. 25(a) can be implemented using the blockdiagram shown in FIG. 4 and results in the bitstream depicted in FIG.22(a) being transmitted. The downhole processor 56 proceeds from step2402 to step 2502 a where it reads raw measurement data from the sensormodules 50,52 and encodes this raw data into an encoded digitalbitstream in the form of the data frame to be transmitted (step 2504 a;the bitstream that includes the data frame is hereinafter the “databitstream”). At step 2506 a the downhole processor 56 determines the ID,digitally encodes the ID as part of the data bitstream, and alsodetermines the content of the long header and when to begin transmittingthe long header and ID. In the bitstream of FIG. 22(a), the data frameis transmitted immediately following transmission of the ID. The ID istransmitted a short delay after completion of transmission of thecurrent data frame. The processing power of the downhole processor 56 istypically limited; consequently, when the EM telemetry signal is sent tothe downhole location, the short delay between the end of the one dataframe and the start of transmission of the subsequent ID gives thedownhole processor 56 sufficient processing time to identify thebeginning of the ID. This short delay is typically not required whentransmitting a signal to surface because surface processing equipment isconsiderably more powerful than the downhole processor 56; in analternative embodiment (not depicted), the short delay may also beomitted when transmitting signals to a downhole location if the downholeprocessor 56 has sufficient processing power. In the embodiment of FIG.25(a), the downhole processor 56 determines when the current data framewill end and schedules the ID to be transmitted on the data channel ashort delay after the current data frame ends. The downhole processor 56then determines, from the scheduled time that the ID is to starttransmitting, when to begin transmitting the long header on the headerchannel by subtracting from the ID's start time the time it will take totransmit the long header. Commencing transmission of the long header atthis time results in transmission of the ID beginning simultaneouslywith transmission of the long header completing. As shown in FIG. 22(a),this results in a portion of the long header being transmittedsimultaneously with the current data frame. The header delay between theend of the current data frame and the start of the next data frame isaccordingly reduced relative to that of FIG. 21(b).

Following determining when to transmit the long header and ID, at step2508 a the downhole processor 56 encodes the long header into a digitalbitstream (“header bitstream”) and converts the data bitstream and theheader bitstream into a data waveform and a header waveform,respectively. This conversion involves using a selected digitalmodulation technique to modulate the waveforms, with the frequency ofeach waveform being modulated being determined in accordance withEquations (1) through (4). At step 2508 a, the downhole processor 56also digitally combines the data and header waveforms into a combinedwaveform. The downhole processor 56 then proceeds to step 2510 a andgenerates control signals to the H-bridge circuit 106, voltageregulators 114, 116, and gap sub 12 to transmit an EM telemetry signalcomprising the combined waveform (otherwise referred to as “carrierwave”) across the gap sub 12. Following generation of the controlsignals, the appropriate voltages are applied across the gap sub 12(step 2512 a) and the EM signal is transmitted to the surface. Oncereceived at the surface by the receiver 80, the EM signal may bedemodulated in accordance with FIG. 7, as described above.

Various digital modulation techniques known in the art can be used toencode each separate waveform, such as ASK, PSK, FSK, BPSK, QPSK or anycombination of these or other individual modulation techniques as isknown in the art. As the frequencies of the separate waveforms are setat different values, the plurality of separate waveforms encoded by oneor more of these techniques can be superimposed to form one combinedwaveform (carrier wave) for transmission to surface. As a result, one EMsignal comprising the carrier wave composed of two or more telemetrychannels can be transmitted to surface.

Referring now to FIG. 25(b), there is shown another embodiment of themethod 2400 for transmitting a frame of the EM telemetry signal. As withthe embodiment of FIG. 25(a), the embodiment of FIG. 25(b) can beimplemented according to the block diagram of FIG. 4. Applying theembodiment of FIG. 25(b) results in the bitstream shown in FIG. 22(b)being transmitted.

As with the embodiment of FIG. 25(a), when performing the embodiment ofFIG. 25(b) the downhole processor 56 moves to step 2502 b from step2402, reads raw measurement data, and then encodes the raw data into thedata bitstream at step 2504 b. As with the bitstream of FIG. 22(a), inthe bitstream of FIG. 22(b) the data frame is transmitted immediatelyfollowing transmission of the ID. The downhole processor 56 proceeds tostep 2506 b where it determines the content of the long header and whento begin transmitting the long header and ID. Unlike the bitstream ofFIG. 22(a), in the bitstream of FIG. 22(b) the current data frame isinterrupted by the ID; this could be done, for example, in response to achange from sliding to rotating drilling or in response to a toolmalfunction. The downhole processor 56 determines a time at which tointerrupt transmission of the current data frame with the transmissionof the ID by determining when the next data packet in the current dataframe ends; instead of then sending the subsequent data packet in theframe, the processor 56 sends the ID. The downhole processor 56 thendetermines, from the scheduled time that the ID is to starttransmitting, when to begin transmitting the long header on the headerchannel by subtracting from the ID start time the time it will take totransmit the long header. Commencing transmission of the header at thistime results in transmission of the ID beginning simultaneously withcompletion of transmission of the long header. As shown in FIG. 22(b),this results in the long header being transmitted simultaneously withthe current data frame. The header delay between the end of the currentdata frame and the start of the next data frame is accordingly reducedrelative to that of FIG. 21(c).

Following determining when to transmit the ID and long header, at step2508 b the downhole processor 56 generates the header bitstream,converts the data and header bitstreams into data and header waveforms,respectively, and combines the two waveforms, as is done at step 2508 a.The downhole processor 56 then generates the control signals used forsignal transmission and transmits the EM signal at steps 2510 b and 2512b, as is done at steps 2510 a and 2512 a. Once received at the surfaceby the receiver 80, the EM signal may be demodulated in accordance withFIG. 7 as described above.

Referring now to FIG. 25(c), there is shown another embodiment of themethod 2400 for transmitting a frame of the EM telemetry signal. As withthe embodiment of FIG. 25(a), the embodiment of FIG. 25(c) can beimplemented according to the block diagram of FIG. 4. Applying theembodiment of FIG. 25(c) results in the bitstream shown in FIG. 22(c)being transmitted.

As with the embodiment of FIG. 25(a), when performing the embodiment ofFIG. 25(c) the downhole processor 56 proceeds to step 2502 c from step2402 where it reads raw measurement data. The downhole processor 56 thenproceeds to step 2504 c where it encodes the raw data into the databitstream at step 2504 c. As with the bitstream of FIG. 22(a), in thebitstream of FIG. 22(c) the data frame is transmitted immediatelyfollowing transmission of the ID. At step 2506 c the downhole processor56 determines the content of the long header and when to begintransmitting the long header and ID. As in the bitstream of FIG. 22(b),in the bitstream of FIG. 22(c) transmission of the current data frame isinterrupted by the ID. The downhole processor 56 determines a time atwhich to interrupt transmission of the current data frame withtransmission of the ID. The downhole processor 56 schedules transmissionof the long header and the ID to start simultaneously. As shown in FIG.22(c), this results in the long header, ID, and data frame beingtransmitted simultaneously. The header delay between the two data framesis accordingly reduced relative to that of FIG. 21(c). Following signaltransmission the downhole processor 56 proceeds to step 2408 where themethod 2400 ends.

Following determining when to transmit the ID and long header, at step2508 c the downhole processor 56 generates the header bitstream,converts the data and long header bitstreams into data and headerwaveforms, respectively, and combines the two waveforms, as is done atstep 2508 a. The downhole processor 56 then generates the controlsignals used for signal transmission and transmits the EM signal atsteps 2510 c and 2512 c, as is done at steps 2510 a and 2512 a. Oncereceived at the surface by the receiver 80, the EM signal may bedemodulated in accordance with FIG. 7 as described above. Followingsignal transmission the downhole processor 56 proceeds to step 2408where the method 2400 ends. In the embodiment shown in FIG. 22(c),transmission of the long header is completed prior to completion oftransmission of the data frame. Consequently, once the data frame hasbeen transmitted, the receiver 80 has sufficient information todemodulate and use the data contained in the frame.

Referring now to FIG. 22(d), there is shown the header and databitstreams transmitted along the header and data channels, according toanother embodiment. The embodiment used to generate the bitstreams shownin FIG. 22(d) is identical to that used to generate the bitstreams shownin FIG. 22(a) except that the entire header portion is transmitted usingthe header channel in place of only the long header in FIG. 22(a).Consequently, only the data frame is transmitted using the data channel.The header portion completes transmission simultaneously with the dataframe commencing transmission. As the data channel is used exclusivelyfor data frames in FIG. 22(d), all other things being equal theeffective data transfer rate of bitstreams shown in FIG. 22(d) is higherthan that of FIG. 22(a). Although not depicted, the long header and IDcan be combined in the bitstreams shown in FIGS. 22(b) and (c) as wellin an analogous manner such that instead of only the long header beingtransmitted on the header channel, the long header and ID aretransmitted on the header channels.

Referring now to FIGS. 26(a)-(f), there are shown examples of waveformsgenerated and recovered by the EM telemetry system 30, in accordancewith the embodiment of FIG. 25(a). FIG. 26(a) shows the data waveformmodulated using BPSK having a carrier frequency of six Hz and with theID and data frame encoded at three cycles/bit. FIG. 26(b) shows theheader waveform also modulated using BPSK having a carrier frequency ofthree Hz and with the header encoded at three cycles/bit. FIG. 26(c)shows the combined waveform resulting from the superposition of the dataand header waveforms. It is the combined waveform that is transmittedthrough the earth and that is received by the receiver 80. The combinedwaveform as received by the receiver 80 is shown in FIG. 26(d).Following filtering and other signal conditioning as described above inrespect of FIG. 7, the data waveform is recovered as shown in FIG. 26(e)and the header waveform is recovered as shown in FIG. 26(f).

Using multiple channels to transmit frames can be used, for example,during LWD when measurements are typically sent a certain number oftimes per foot drilled. If, for example, the EM signal transmitter 13 istransmitting two frames per foot drilled, it takes twenty seconds totransmit the header, and drilling is occurring at the rate of one footevery twenty seconds, then measurement data cannot be sent in real timeto the receiver 80 while drilling is occurring. However, the datatransfer rate can be increased by sending the long header on a separatechannel from the data frame, which allows the long header to beshortened and the long headers and data frames to be transmittedsimultaneously.

In another embodiment, the downhole processor 56 can take advantage ofidle periods to transfer data, as illustrated using FIGS. 23(a)-(c) andin accordance with a method 2412 shown in FIG. 24(b). FIG. 23(a) shows agraph of mud being pumped downhole vs. time. Prior to t₀, mud is pumpeddownhole at a constant rate. From t₀ to t₁, mud flow ceases and the BHAdoes not perform a survey; this period during which no mud is pumped andno survey is performed is referred to as an “idle period”, which is aperiod during which vibration is allowed to dampen so as not tointerfere with surveying. A typical duration for the idle period isapproximately thirty seconds. A conventionally configured downholeprocessor 56 does not transmit any signals to surface during the idleperiod; this is illustrated in FIG. 23(b). Following the idle period,from t₁ to t₂ in FIG. 23(b), the BHA acquires a survey; this period isaccordingly referred to as a “survey acquisition period”. Aconventionally configured downhole processor 56 begins transmitting theframe after the survey acquisition period, also as shown in FIG. 23(b).Following the survey acquisition period mud pumping restarts.

Referring now in particular to the method 2412 of FIG. 24(b) andbitstreams (i) and (ii) of FIG. 23(c), the downhole processor 56according to one embodiment may commence transmitting during the idleperiod, thereby transmitting the data acquired during the survey morequickly than when this data is conventionally sent, as depicted in FIG.23(b). The method 2412 of FIG. 24(b) transmits a data frame of the EMtelemetry signal from a downhole location to surface. The downholeprocessor 56 begins performing the method 2412 at step 2414, followingwhich it proceeds to step 2416 where it transmits at least part of thelong header of the EM telemetry signal during the idle period. This isillustrated in waveform (i) of FIG. 23(c) when only the long header istransmitted during the idle period, and in waveform (ii) of FIG. 23(c)when the entire header portion (i.e., the long header and the ID) istransmitted during the idle period. The downhole processor 56 determineswhen to begin transmitting the long header by subtracting from t₂ thetime it will take to transmit the long header (for waveform (i)) or theentire header portion (for waveform (ii)). Following transmission of thelong header (for waveform (i)) or the header portion (for waveform(ii)), the downhole processor proceeds to step 2418 where it acquires asurvey during the survey acquisition period, which immediately followsthe idle period. As the survey is being acquired, the downhole processor56 encodes the results of the survey into the data frame at step 2420.The downhole processor 56 then proceeds to step 2422 where it transmitsthe data frame and any portion of the long header not transmitted duringthe idle period; in FIG. 23(c), only the data frame is transmitted tothe surface following the survey acquisition period. Followingtransmission, the processor 56 proceeds to step 2424 where the method2412 ends.

The controller used in the foregoing embodiments may be, for example, amicroprocessor, microcontroller, digital signal processor, programmablelogic controller, field programmable gate array, or anapplication-specific integrated circuit. Examples of the computerreadable medium are non-transitory and include disc-based media such asCD-ROMs and DVDs, magnetic media such as hard drives and other forms ofmagnetic disk storage, semiconductor based media such as flash media,random access memory, and read only memory.

It is contemplated that any part of any aspect or embodiment discussedin this specification can be implemented or combined with any part ofany other aspect or embodiment discussed in this specification.

For the sake of convenience, the exemplary embodiments above aredescribed as various interconnected functional blocks. This is notnecessary, however, and there may be cases where these functional blocksare equivalently aggregated into a single logic device, program oroperation with unclear boundaries. In any event, the functional blockscan be implemented by themselves, or in combination with other pieces ofhardware or software.

While particular embodiments have been described in the foregoing, it isto be understood that other embodiments are possible and are intended tobe included herein. It will be clear to any person skilled in the artthat modifications of and adjustments to the foregoing embodiments, notshown, are possible.

The invention claimed is:
 1. A method for transmitting at least aportion of a data frame of an electromagnetic (EM) telemetry signal froma downhole location, the method being performed in an electronicssubassembly communicative with an EM carrier frequency signal generatorand comprising a digital encoder having a processor and a memorycontaining encoder program code executable by the processor, the methodcomprising: (a) encoding, using the digital encoder, at least a portionof a long header into a header waveform having a first frequency,wherein the EM telemetry signal comprises the long header and the atleast a portion of the data frame, and wherein the long header isindependent of a content of the data frame, and is used to synchronizecommunications between an EM signal transmitter and an EM signalreceiver; and (b) encoding, using the digital encoder, the at least aportion of the data frame and any remaining portion of the long headerinto a data waveform having one or more additional frequencies differentfrom each other and from the first frequency, wherein the firstfrequency and the one or more additional frequencies are sufficientlyseparated to permit demodulation of the at least a portion of the longheader encoded into the header waveform and the at least a portion ofthe data frame and any remaining portion of the long header encoded intothe data waveform; (c) digitally combining, using the digital encoder,the header and data waveforms into a combined EM carrier waveform; and(d) generating, using the EM carrier frequency signal generator, thecombined EM carrier waveform across an electrically isolated gap of thegap sub assembly; and (f) transmitting the combined EM carrier waveform.2. The method of claim 1 wherein all of the long header is transmittedat the first frequency and all of the data frame is transmitted at asecond frequency.
 3. The method of claim 2 whereinf′(1+(1/c′))□f(1−(1/c)), wherein f is one of the first and secondfrequencies, f′ is the other of the first and second frequencies,f>f, cis cycles per bit transmitted at f, and c′ is cycles per bit transmittedat f′, and wherein c>1 and c′>1.
 4. The method of claim 3 wherein c=c′.5. The method of claim 3 wherein f′ is the first frequency and f is thesecond frequency.
 6. The method of claim 2 further comprisingtransmitting an identifier (ID), which varies with the content of thedata frame, at the first or second frequencies.
 7. The method of claim 6wherein all of the ID and all of the data frame are transmitted at thesecond frequency.
 8. The method of claim 7 wherein the long header istransmitted prior to the ID.
 9. The method of claim 8 wherein at least aportion of the long header is transmitted simultaneously with a previousdata frame.
 10. The method of claim 9 wherein all of the previous dataframe is transmitted prior to transmission of the ID.
 11. The method ofclaim 9 wherein less than all of the previous data frame is transmittedprior to transmission of the ID.
 12. The method of claim 10 wherein theprevious data frame and the long header complete transmittingsimultaneously.
 13. The method of claim 7 wherein at least a portion ofthe long header is transmitted simultaneously with the ID.
 14. Themethod of claim 13 wherein the long header completes transmitting duringtransmission of the at least a portion of the data frame.
 15. The methodof claim 6 wherein all of the long header and the ID are transmitted atthe first frequency and all of the data frame is transmitted at thesecond frequency.
 16. The method of claim 15 wherein all of a previousdata frame is transmitted prior to transmission of the at least aportion of the data frame.
 17. The method of claim 15 wherein less thanall of a previous data frame is transmitted prior to transmission of theat least a portion of the data frame.
 18. The method of claim 16 whereinthe previous data frame that is transmitted and the ID completetransmitting simultaneously.
 19. The method of claim 6 wherein the atleast a portion of the data frame is transmitted from the downholelocation, and further comprising pausing between transmitting the ID andthe at least a portion of the data frame.
 20. The method of claim 1wherein the at least a portion of the data frame is transmitted from thedownhole location and sent to surface.
 21. The method of claim 20wherein the data frame comprises data obtained during a survey andwherein transmission of the at least a portion of the long headercommences during an idle period immediately prior to acquiring thesurvey.
 22. The method of claim 1, wherein the at least a portion of thelong header is encoded according to a first modulation technique, andwherein the at least a portion of the data frame is encoded according toa second modulation technique different from the first modulationtechnique.
 23. A non-transitory computer readable medium having encodedthereon statements and instructions to cause a processor to perform themethod of claim
 1. 24. A downhole telemetry tool for transmitting aframe of an electromagnetic (EM) telemetry signal from a downholelocation, the tool comprising: (a) a gap sub assembly; (b) an EM carrierfrequency signal generator for generating an EM carrier wave across anelectrically isolated gap of the gap sub assembly; (c) an electronicssubassembly communicative with the signal generator and comprising adigital encoder having a downhole processor and a memory containingencoder program code executable by the downhole processor to perform themethod of claim 1.